158 Scientific Proceedings, Royal Dublin Society. 
_ This is true for both isotropic and anisotropic crystals, the 
refractive index in the latter being taken as the mean of the 
different indices corresponding to the three chief axes of elasticity. 
But since the law of Gladstone and Dale holds good when » is 
taken as the mean of the indices in a doubly refracting crystal, 
we may naturally inquire what signification it can have for the 
maximum, mean, and minimum indices. 
Ny — 1 
If in the case of a uniaxial crystal we put i, oe 
oo ae 
ne -1 
ae 
indices, respectively, three suppositions are possible. «, and Km 
may have identical values, and then d, and d, must be different, 
i.e. the density of the crystal must be different in different 
directions; those, namely, of the optic axis and equatorial plane, 
or d, and d,, may be identical, in which case x, and x, must 
vary directly as the refractive powers, or the specific refractive 
energy must be different in different directions; this supposi- 
tion, like the last, leading to the conclusion that the constituent 
chemical atoms are differently arranged in different directions. 
= ko, 2 and n, standing for the maximum and minimum 
Finally, we may suppose that both « and d vary, and then 
both the previous conclusions will be true; the density and 
chemical composition will both differ, according to direction in 
the crystal. 
But if the constancy of the specific refractive energy or of the 
refractive equivalents of the elements be maintained in an aniso- 
tropic crystal, as we have seen it is, then the possibility suggests 
itself of using this constancy as a means of exploring the molecu- 
lar structure of the crystal; in other words, of using Gladstone 
and Dale’s law as an optical probe. 
As an example, we may select the two similarly constituted 
salts, potassium and sodium nitrates, the latter crystallizing in 
rhombohedra of the hexagonal system; the former likewise in 
rhombohedra, but also in forms belonging to the rhombic system : 
the refractive indices of potassium nitrate, however, are only 
known for the rhombic forms. The optic sign is in both cases 
negative, and the refractive indices are as follows :— 
Potassium nitrate, my, 1°5052 mn, 15046 n, 1:3338; 
Sodium nitrate, . N. 1:586 Nn. Ieogaae 
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